In a piston ring groove provided in the outer periphery of a piston of an internal combustion engine, a piston ring is installed. A configuration of the piston ring in the ordinary gasoline internal combustion engine is composed of two compression rings (a top ring and a second ring) and a combination oil ring. The combination oil ring has an oil control function to suppress the consumption of an engine oil.
As a prior art literatures relating to such an oil ring, the followings are exemplified. PTL 1 discloses, in two discs (side rails) of the oil ring having running surfaces in an asymmetrical convex shape, an oil scraping ring whose vertex lines of the outer peripheral surfaces of the discs being directed in opposite directions toward the middle of the ring groove, and an oil scraping ring whose vertex lines of the outer peripheral surfaces of the discs being directed in the same direction toward the side surface in the opposite direction to the piston top of the ring groove. Further, it is also disclosed that the transverse section of the running surface of the disc is in an asymmetrical shape of a quadratic polynomial expressed by h(x)=ax+bx2 in a first section, and in an asymmetrical shape of a function of h(x)=cx2 in a third section, after a supporting vertex (II)h(x=0) configured as an edge.
Further, PTL 2 discloses a two-piece type combination oil ring having upper and lower rails and an expander. PTL 2 discloses that each of outer peripheral surfaces of the upper and lower rails is formed by an asymmetric barrel curve with a position closer to the lower part in the axial direction from the center of the width in the rail axial direction as a vertex, and the drop in the radial direction on the upper side of the asymmetric barrel curve is preferably 25 to 75 μm and the drop in the radial direction on the lower side of the asymmetric barrel curve is preferably 1 to 20 μm.
Further, NPL 1 discloses a configuration that the outer peripheral surface shape of the segment (side rail) is provided in a symmetrical convex shape in actual operation of a 2-liter 4-cycle gasoline engine. NPL 1 states an oil film thickness on a contour curve of the segment (side rail) when supplying to a test a 3-piece oil ring having a drop from the outer peripheral vertex of a measurement width of 0.15 mm in a slide direction of the segment (side rail) on the order of about 4 μm. It is studied and reported (FIG. 12) that the oil film thickness is about 1 μm to 6 μm in four cycles on a thrust side under a full-load operation condition of 2000 rpm by the LIF (Laser-Induced-Fluorescence) method using fiber. The fact that the oil film thickness increases with an increase in the number of revolutions of the engine is described in the report.
Further, in NPL 2, it is studied and reported (FIG. 10) that the oil film thickness of the 3-piece oil ring (an oil ring D) is about 1 μm to 4 μm in four cycles on the thrust side under an operation condition of 2000 rpm and 75% 1-Load during actual operation of a 0.3-liter single cylinder 4-cycle test diesel engine.